Hydraulic holding valves and counterbalance valves are well known in the art. Such valves are functionally similar and both are used to control an overrunning or overhauling load in a hydraulic system. A holding valve may be characterized simply as a "zero leak" counterbalance valve. Thus, a holding valve may, for example, be used to maintain the load carrying boom of a crane, operated by a hydraulic cylinder, in a fixed position, where any downward movement through fluid leakage and resultant retraction of the cylinder would be undesirable or dangerous.
In the prior art, holding valves and counterbalance valves utilize a spring to bias the valve closed against the hydraulic pressure being held in the cylinder or other hydraulic actuator. A holding valve of this type is opened for releasing the load by applying hydraulic pressure, in addition to the system pressure being held, sufficient to overcome the spring force holding the valve closed. This additional pressure is generally supplied by pilot pressure produced by reversing the system flow to the cylinder to release the load. The bias spring must obviously be strong enough to hold the valve closed against a pressure somewhat greater than the maximum desired system holding pressure and, thus, at low system pressures, the added pressure required to overcome the spring force and open the valve is high. Much more energy is therefore required to provide pilot opening pressure at low load pressures than at high pressures. Furthermore, since the bias spring is the only means by which the valve is held closed and the load holding pressure maintained, spring failure will render the valve inoperative and, if the spring should fail when a load is being held, serious damage or injury could result. Also, at high load holding pressures, leakage and loss of holding pressure is more likely to occur simply because that pressure acts against a constant spring force.